This invention relates to the field of production of industrial gases. In particular, the invention includes a method and apparatus for producing ultra-pure gaseous nitrogen by non-cryogenic means.
Various industrial processes require the use of ultra-pure nitrogen, i.e. gaseous nitrogen having an oxygen content in the range of 1-5 parts per million (ppm), by volume. An example of such a process is a high-temperature heat treating furnace, wherein it is important to provide a substantially oxygen-free atmosphere. One way of providing ultra-pure nitrogen is to obtain and store it in liquid form, and to vaporize it immediately prior to use. Such a procedure yields nitrogen gas having an oxygen content of about 10 ppm or less, but requires equipment for storing and handling the cryogenic liquid which cannot be economically justified for small users.
It has also been known to provide nitrogen using a non-cryogenic process. In principle, one can obtain nitrogen directly from ambient air, simply by passing the air through a molecular sieve to remove most of the oxygen. This task is typically performed by a commercially-available device known as a pressure swing adsorption (PSA) nitrogen unit. In the PSA unit, incoming air is compressed and forced through a bed of molecular sieves which remove most of the oxygen from the air. The result is a product comprising mainly nitrogen, and having an oxygen content of about 1% by volume. One can obtain even better results from a PSA unit, but to do so requires significantly greater capital and operating costs. Alternatively, it is possible to remove most of the oxygen remaining in the stream exiting the PSA unit through the use of a deoxygenating device (also known as a "deoxo" unit).
Deoxygenating devices have been known in the prior art. Examples are described in U.S. Pat. Nos. 5,122,355 and 5,318,759. The latter patents discuss the use of catalysts to remove residual oxygen from the nitrogen stream. However, U.S. Pat. No. 5,318,759 produces a product which is not considered oxygen-free, and U.S. Pat. No. 5,122,355, while producing a pure product, requires either the expenditure of considerable energy, or requires the use of expensive equipment, or both. The disclosures of the above-cited patents are hereby incorporated by reference into this specification.
While it is possible to produce ultra-pure nitrogen using the systems of the prior art, such systems are usually not economical unless the production rate is at least about 10,000 scfh (standard cubic feet per hour). The systems of the prior art are expensive, both with respect to the energy needed to operate them, and with respect to the cost of the components and the piping and instrumentation connecting them. Such systems cannot justify themselves economically for smaller rates of production.
The present invention provides a method and apparatus for producing ultra-pure nitrogen, using a very cost-efficient non-cryogenic technique. The present invention is especially useful, and economically attractive, in applications which do not require large flow rates of nitrogen. In particular, the present invention provides an economical way of making ultra-pure nitrogen in systems requiring nitrogen at flow rates in the range of about 100 scfh to about 9000 scfh. The nitrogen produced by the present invention is substantially oxygen-free, having an oxygen content of less than 10 ppm by volume, and has a dew point of -70.degree. F. or lower.